1. Field of the Invention
This invention relates to the transfer of nuclear fuel between storage pools and shipping casks. More particularly, the invention relates to an improved method and apparatus which substantially reduces the probability of nuclear contamination during the transfer of nuclear fuel elements between a fluid-filled storage pool and a shipping cask.
2. Description of the Prior Art
In the operation of nuclear reactors, a controlled nuclear chain reaction is maintained in a reactor core by fuel elements containing radioactive uranium. Typically, these elements comprise long, thin, tubular structures made of steel, clad with a zirconium alloy and within which are packed a number of pellets containing a radioactive uranium composition. The fuel elements which can be efficiently utilized for extended periods of time eventually fail due to reduced activity or physical integrity. The resulting spent fuel elements must then be removed and replaced to assure safe, efficient reactor operation.
After removal from the reactor core, the spent fuel elements are generally transferred to a fluid-filled fuel storage pool for retention pending shipment to a disposal or recycling facility. Similarly, fresh fuel elements can be stored in fluid-filled pools after shipment to the nuclear reactor location but prior to their placement in the core. Spent fuel elements are typically shipped from one point to another in sealed, fluid-filled, shielded containers called casks. Transfer of the fuel elements from the pool to the casks, and from the casks to the pool, must usually be done without removing the fuel from the fluid, using constantly-filtered water, to assure maximum safety. However, the art has experienced substantial difficulty in safely and efficiently effecting such transfer. Water employed in the pool or gas or air surrounding the spent fuel will be contaminated regardless of precautions taken. Moreover, even if a system were designed to have essentially zero contamination in the environment, prudence would still dictate treating it as if it were contaminated to guard against possible anomalies in the system.
Among the early prior art transfer systems were those which immersed the cask in the pool to allow transfer without removing the fuel from the water. There were many risks attendant with such transfer systems, not the least of which was the total wetting of the cask exterior with contaminated water. The contaminated wash water produced had to be disposed of.
Faced with this problem of cask contamination, there have evolved a number of systems for effecting transfer without wholly immersing the cask. These systems have come to be known as dry cask systems. Unfortunately, known dry cask systems have exhibited various drawbacks.
For example, U.S. Pat. No. 3,765,549 presents a system employing a pair of independently-actuatable, concentric bellows mounted beneath a fuel storage pool and circumscribing a hatch therein. According to that disclosure, a fuel cask is positioned directly below the hatch and the bellows are extended downwardly to seat against the cask and form a transfer channel between the pool and the cask. The channel is then flooded, the hatch opened, and transfer effected. The particular arrangement of bellows and supporting devices shown, however, render the system susceptible to serious losses of contaminated material in the event the cask to bellows sealing surfaces do not match perfectly or if the cask sealing surface becomes dirty or damaged in transportation. This system does not provide secondary means for preventing leakage and is susceptible to leakage in the event of moderate seismic disturbances.
In an attempt at overcoming certain of the difficulties of the bellows arrangements, U.S. Pat. No. 3,910,006 discloses that a direct contact between a cask and the underside of a transfer pool can be employed. This arrangement is said to eliminate the problems associated with large differential pressures on the bellows and the large amounts of water that the bellows arrangements must employ in the transfer channel. Here again different casks with different fabrication tolerances have to be matched with sealing surfaces beneath the fuel transfer pool and leakage cannot be prevented if casks do not properly match the surface or if their sealing surfaces become dirty or damaged in transportation. This system does not provide a secondary means for preventing leakage and is also susceptible to leakage in the event of a moderate seismic disturbance.
In U.S. Pat. No. 3,883,012 there is described yet another dry cask system. In particular it is disclosed that the fuel cask can be positioned within a tank to accommodate casks of varying dimensions and to avoid some of the risks that might still surround the use of systems such as that described in U.S. Pat. No. 3,765,549. While this disclosure suggests lateral seismic restraints on the cask tank, no means are identified for suitably positioning the cask transporter and for restraining the cask tank while at the loading terminal.
The art has thus evolved fuel transfer systems culminating in a number of dry cask systems. There remains however, a need for a dry cask system which permits safe and efficient transfer of nuclear fuel elements between casks and fluid-filled storage pools.